As integrated circuit feature sizes continue to shrink, new low dielectric constant (low-k) materials are needed to address problems with power consumption, signal propagation delays, and cross talk between interconnects. One avenue to low-k dielectric films is introduction of nanometer scale pores to lower its effective dielectric constant. However, the pore structure strongly affects important material properties such as mechanical strength, moisture uptake, thermal expansion, and adhesion to different substrates. Therefore, characterization of the pore structure, in particular the pore size distribution and mechanical properties, is strongly needed to optimize and develop new low-k materials and processes.
Traditional methods used for the porosity characterization in bulk materials are hardly applicable to thin films because the total pore volume and surface area are too small. For this reason, advanced non-destructive methods, such as small-angle neutron and X-ray scattering (SANS and SAXS) combined with specular X-ray reflectivity (XRR) and positron annihilation spectroscopy (PALS, PAS) have recently been developed to characterize the pore size and porosity of thin porous films. Although these new techniques are based on different physico-chemical principles, few systematic studies reported so far show that the results of the measurements are in reasonable agreement.
Low stiffness properties of porous low-K films is one of key factors limiting their introduction into ULSI technology. A compromise must be reached between low dielectric constant and sufficient mechanical strength for the material to survive technological steps. There is also lack of useful and accessible techniques, which can accurately provide absolute values of the mechanical characteristics.
In PALS and PAS, films are irradiated with a focused beam of several keV positrons. Positrons form positronium (Ps)xe2x80x94the electron-positron bound statexe2x80x94that is trapped in the pores where their natural lifetime of 142 ns is reduced by annihilation during collisions with the pore walls. The reduced lifetime xcfx84(Ps) can be correlated with pore size. Ps lifetime histograms are recorded, and the lifetime distribution curves are obtained with a fitting program specified for this purpose. The distribution curves are transformed into pore size data, using pore geometries. The film porosity can be calculated by comparison of measured photon annihilation ratio of Ps atoms. In PALS, the porosity characterization needs deposition of a special barrier to compare the Ps intensity in free and capped films. PAS and PALS are efficient for the evaluation of bi-modal pores (like MSSQ). If pores are bi-modal, they give information related to their size and relative concentration. PALS and PAS are useful for characterization of pore interconnectivity and can be used for evaluation of diffusion barriers by detecting of Ps escaping from the film trough the voids in the barrier. However, because xcfx84(Ps) also depends on the wall nature, sometimes it is difficult to obtain the pore size from xcfx84(Ps): for instance, in the case of organic polymers. If all pores are open, one needs to apply a capping layer: otherwise Ps escape to vacuum and give the natural xcfx84=142 ns.
In SANS, the absolute scattered neutron intensity, I, plotted against the scattering vector q=(4xcfx80/xcex)sin(xcex8/2) where xcex8 is the scattering angle from the incident beam path and xcex is the neutron wavelength (6 xc3x85). The SANS intensity plotted versus q is a function of the porosity and wall density. The functional form is determined assuming a random two-phase (void+solid) structure. The film thickness and overall electron density are evaluated by the XRR measurements and are combined with the film composition data obtained by RBS and FRES so that the overall film density is determined. Since the film density is also a function of the porosity and skeleton density, these values are obtained by solving for the unknowns in the equations from SANS and XRR.
Recently, a simple X-ray scattering method for thin film evaluation was reported. The pore size is calculated by comparing the observed profile of scattering intensity and results of simulation. This approach is convenient to get general information without details because only effective pore size is calculated. If the film has bi-modal pores, the effective pore size depends on the ratio between small and large pores. For instance, if they have the same volume, the SAXS pore size is closer to the small pores due to the larger number of interfaces. The film porosity is calculated by normalization of the XRR film density to the skeleton density. This necessitates the assumption that the skeleton is identical to the dense, non-porous prototype. Sometimes such an assumption is not justified. The non-porous prototype may also be not available (for instance, in the case of CVD SiOCH films). This method is not efficient for evaluation of the pores interconnectivity, for evaluation of diffusion barriers and, generally, SAXS is not able to distinguish between pores and particles.
Nanoindentation (NI) is the most common method for obtaining stiffness of thin films. However, NI overestimates stiffness because of several possible reasons: (a) Stiffening by the substrate. For such thin films the NI tip may always feel the effect of the substrate and thus overestimate Young""s Modulus (E); (b) Viscoelasticity. Polymers are known to show large viscoelastic effects which are likely to cause higher E values to be obtained; (c) Tip-film interactions. Effects such as densification or pile-up under the tip have not been quantified. The interactions of a tip with such a porous matrix are not well understood. Additionally, NI is destructive and therefore it is not applicable for in-line monitoring of low-k films.
Two different non-destructive methods have recently been successfully used for evaluation of stiffness properties of porous low-K films. Results of the stiffness measurements of MSSQ based porous low-K films by Surface Acoustic Wave Spectroscopy (SAWS) and Brillouin Light Scattering (BLS) are in good agreement one with another but E values calculated for various low-K films are ≈3 times lower than NI. The film density and porosity calculated from the same SAWS data correlate excellently with Specular X-Ray Reflectivity. These facts suggest that the E values obtained by SAWS and BLS are real and accurate.
In the SAWS method for non-destructive characterization of density and Young""s Modulus of low-k films, surface acoustic wavepackets are generated thermoelastically from absorption of laser pulse energy at the layer/substrate interface. The laser pulse energy (337 nm wavelength) is focused into a thin line on the sample, and causes rapid expansion of the locally heated source, giving rise to stresses and generating surface acoustic wavepackets propagating along the sample. The wideband SAW wavepackets are detected by a piezoelectric foil with a steel-wedge transducer at different relative propagation distances (here 15 mm) on the sample. The broadband SAW wavepacket (approx. 20-100 MHz frequency range) propagates in both layer and substrate and becomes dispersed because waves of different frequency sample a different proportion of layer and substrate, with different net elastic properties, and the wave velocity is therefore frequency dependent. From a Fourier transform technique one extracts the frequency-dependent velocity dispersion curve. Assuming that thickness and Poisson""s ratio are known, the density and Young""s modulus of the layer are obtained from the best-fit parameters of the theoretical to the measured dispersion curve. The SAWS film density is in good agreement with XRR. Although SAWS is not able to measure the pore size, a unique feature of this method is the possibility of non-destructive evaluation of mechanical properties (Young""s Modulus).
It is the aim of the invention to provide a method and apparatus for evaluation of films, more especially low-k thin films with nano-scale pores.
In a first aspect thereof, said evaluation may include characterization of the pore structure, said characterization results in determining pore sizes, hence obtaining pore size data.
In a second aspect thereof, said characterization may result in non-destructive evaluation of mechanical properties, in particular the Young""s Modulus.
In a third aspect of the invention, a method is provided which is suitable for both in line monitoring or studying of pore structure porosity and pore size distribution (PSD) of low-k films and evaluation of the mechanical properties of porous low-k films simultaneously using the same set of experimental data.
Said method and apparatus for characterization can provide information useful for the development of new types of low-k dielectrics and optimisation of a variety of technological processes, for instance in the phase of their integration but also in quality control testing and process control.
Said method and apparatus is based on ellipsometric porosimetry, (also called EP), which evaluates optical characteristics of a porous film during the vapor adsorption in the pores. The proposed characterization does not need complicated calculations and therefore does not need complex arithmetical processors nor large memories. Said method and apparatus uses data obtained by ellipsometric porosimetry.
In a first aspect of the invention, a method for characterizing the pore structure of a film from ellipsometric measurements is disclosed. Said method determines a computed pore size distribution, meaning at least the amount of pores of at least two different sizes, of the pores of a film or just a rough approximation thereof from said ellipsometric porosimetry measurements.
In a first embodiment thereof, a first determination on the quality of said film, in terms of presence of pore-killers, is made based on said computed pore size distribution. In a further embodiment thereof said first determination is used for either making the film sample under investigation a candidate for rejecting in a quality control setting or suggesting possible required adaptations of the film production process parameters are determined in an on-line process control environment. In a second embodiment said first determination is verified by using the same ellipsometric porosimetry measurements, more in particular the thickness of said film. In case it is clear that no film swelling is present, said first determination is confirmed. Then film being candidate for rejection are finally rejected or the need for process parameter changes is confirmed.
Porous films may be prepared by several methods including Spin-on-glass technology and CVD.
The invented method comprises the steps of performing ellipsometric measurements on a film with pores with various pore sizes and computing from said measurements at least an approximation or an indication of the pore size distribution. Said ellipsometric measurements result in data of the refractive index (first data set) and the thickness (second data set) of said film as function of the pressure within the pressurized chamber wherein said film is placed for measuring. Said computation can exploit steps of determining a first slope of the curve defined by parts of the first data set and a second slope by parts of the second data set. The indication of the possible presence of pore-killers relies on said first slope, for instance when said first slope exceeds a first threshold value. Confirmation of said presence is performed when said second slope is below a second threshold value.
In an embodiment of this invention, a method as recited in the previous embodiments is disclosed wherein said film is a porous film used as insulating layer in semiconductor processing.
In an embodiment of this invention, a method as recited in the previous embodiments is disclosed wherein said ellipsometry measurements are performed according to patent application WO 00/1299 and U.S. Pat. No. 6,319,736.
In an embodiment of this invention, a method as recited in the previous embodiments is disclosed wherein said method is for quality testing said film. In an embodiment of this invention, a method as recited in the previous embodiments is disclosed further comprising the steps of accepting or rejecting the film.
In an embodiment of this invention, a method as recited in the previous embodiments is disclosed further comprising the step of changing the parameters of the film fabricating process in response to any determination of pore size or a mechanical property according to the present invention. In an embodiment of this invention, a method as recited in the previous embodiments is disclosed wherein said method is used in a process control unit, said process control unit being for controlling the fabrication process of porous layers.
In a second aspect of the invention, a method for determining the Young""s modulus of a film, placed in a pressurized chamber, comprising the steps of determining via ellipsometric measurements a set of data relating to the change of thickness of the film versus said pressure in said chamber; and calculating the Young""s modulus of said film from said set of data. Alternatively formulated this aspect of the invention is a method for determining the Young""s modulus of an element which is positioned in a pressurizable chamber, filled with a gaseous substance, said method comprising the steps of performing ellipsometric measurements on said element at a pressures being less than equilibrium vapor pressure of said gaseous substance, to determine data on the film thickness as function of said pressure; and determining the Young""s modulus from said data.
In an embodiment of the invention a method as recited in the second aspect of the invention is described wherein said film is a porous film. In a further embodiment of this second aspect of the invention, a method as recited in any of the embodiments is disclosed wherein said film is a porous film used as insulating layer in semiconductor processing.
In an embodiment of this second aspect of the invention, a method as recited in any of the previous embodiments is disclosed wherein said first step is performed according to patent application WO 00/1299 and granted U.S. Pat. No. 6,319,736, which are incorporated herein by reference in their entirety.
In an embodiment of this second aspect of the invention, a method as recited in any of the previous embodiments is disclosed wherein said method is used for quality testing said film. In a further embodiment thereof said quality testing method comprises the steps of accepting or rejecting the film, e.g. in accordance with a threshold value.
In an embodiment of this second aspect of the invention, a method as recited in any of the previous embodiments is disclosed wherein said method is used as part of an on-line process control method for controlling the fabrication process of said film. In a further embodiment thereof said process control method comprising the steps of changing the parameters of the film fabricating process. Fabrication process parameters of the spin-on-glass technology which are changeable via the process control unit are, for example, spinning rotation speed, the length of the heating substeps and their respective heating temperature. Fabrication process parameters of the CVD film making process which are changeable by the process control unit are, for example, the length and temperature of the annealing step.
Alternatively the invention is available as program storage devices readable by a machine and encoding a program of instructions for executing the above described methods. Instruction may be provided for loading data, obtained by performing at least one ellipsometric measurement, computing steps on said data and outputting computed information of said film.